Ind. Eng. Chem. Res. 1988,27, 1873-1882
1873
SEPARATIONS Separation of Sugars by Continuous Annular Chromatography Arnold J. Howard' and Giorgio Carta* Department of Chemical Engineering, University of Virginia, Charlottesuille, Virginia 22901
Charles H. Byers Chemical Technology Division, Oak Ridge National Laboratory,$ Oak Ridge, Tennessee 37831
Continuous separations of mixtures of fructose, glucose, and sucrose have been investigated by using a laboratory-scale continuous annular chromatograph (CAC) with the calcium form Dowex 50W-X8 ion-exchange resin as adsorbent. Comparative chromatographic separation studies have also been conducted for the system using a conventional fixed-bed column packed with the same resin. Complete resolution of fructose-glucose mixtures could be obtained both in a 60-cm-long CAC and in a conventional column of the same length with sugar feed concentrations up t o 200 g/L. In a four-component mixture [blue dextran (higher molecular weight saccharide), sucrose, glucose, and fructose], complete resolution of all species except sucrose-glucose was obtained. The experimental results have been analyzed in terms of approximate linear chromatographic theories for fixed and rotating beds. Bed properties and equilibrium and mass-transfer parameters used in the model were obtained through independent experiments. With these parameters, a good fit to the experimental results was obtained. Differences in feed mixtures and dispersion characteristics contribute to the minor offset between fixed bed and continuous chromatograph results. Liquid chromatography in its various forms has acquired in recent years an important role in a variety of industrial applications. While uses of chromatography for the purpose of analysis have been known since the 1800s and extensive applications exist, industrial scale uses have been much more limited until recently. Advances in the development of chromatographic media and equipment have resulted in effective preparative and production scale separations, involving such techniques as adsorption, ion exchange, affinity, gel filtration, and high-performance liquid chromatography. Although the chemistry and the efficacy of these various techniques differ considerably, the underlying process concept is the same: a sample of the mixture of components that are to be separated is applied to a chromatographic column and the components are separated along the column length as they migrate at different velocities under the effect of an eluent which is continuously supplied to the column. The different velocities that the mixture components assume result from different specific affinities for the sorbent in the chromatographic column. The separated components are eventually recovered at the column exit end as they emerge at different times, and the sample application-elution cycle is repeated. Column chromatography is, thus, inherently a batch process. Continuous operation of separation processes is often advantageous and more efficient for applications at the production scale, especially when the separation system
* Author to whom correspondence should be addressed.
+ Current address: Department of Chemical Engineering, University of Florida, Gainesville, FL 32611. f Operated by Martin Marietta Energy Systems, Inc.,for the US. Department of Energy, under Contract DE-ACOB840R21400.
0888-5885/88/2627-l873$01.50/0
is coupled upstream and/or downstream to other continuous processes. In the past, considerable effort has been dedicated to the development of continuous chromatographic systems. Of the many experimental systems investigated, one in particular has been especially successful: the continuous annular chromatograph (CAC). The apparatus consists of an annular bed of adsorbent particles, packed in the space between two concentric cylinders, as illustrated in Figure 1. While the column assembly is slowly rotated about its axis, eluent and feed solutions are continuously fed into one end of the annular bed. In isocratic operation, the eluent is uniformly fed to the entire bed circumference at the top of the annular bed, while the feed mixture is introduced into a narrow sector of the annulus at a point that remains stationary in space. As time progresses, helical component bands develop from the feed point, with slopes dependent upon eluent velocity, rotational speed, and the distribution coefficient of the component between the fluid and adsorbent phases. At steady state, the component bands form regular helices between the feed sector at the top of the bed and the individual fixed exit points a t the bottom of the annular bed, where the separated components are continuously recovered. As long as conditions remain constant, the angular displacement of each component band from the feed point will also remain constant. The portion of the bed that is not receiving feed at any given time is undergoing either elution or regeneration. Thus, the rotating annular chromatograph is a truly continuous, steady-state process that retains the very attractive characteristics of effectiveness for multicomponent separations and flexibility typical of chromatography. Briefly, the CAC replaces the length and time coordinates characteristic of a conventional chromatographic separation with the length and annular displacement co0 1988 American Chemical Society
1874 Ind. Eng. Chem. Res., Vol. 27, No. 10, 1988 FIXED INLET
*I
I/
Figure 1. Conceptual view of the CAC apparatus.
ordinates of the rotating bed. In this respect, conventional chromatography and CAC are completely analogous, and, in principle, CAC should be able to perform continuously any separation that conventional chromatography can perform in a batchwise fashion. In practice, of course, there may be some limitations associated with flow distribution, materials of construction in relation to their compatibility with the feed and eluent, high-pressure operations, very large scale of operation, and three-dimensional dispersive effects in the annular bed. A number of investigators have studied the development of continuous chromatographic systems for use in industrial scale separations. Martin (1949) was apparently the first to conceptualize an annular chromatograph rotating with respect to a continuous feed stream and product collection points. Giddings (1962) noted that a larger capacity is attainable in an annular space with no loss in resolution when compared to that of a conventional column. Dinelli et al. (1962) were the first to report the assembly of a semicontinuous chromatographic unit arranged as a carousel of 100 conventional columns. Since the initial development of the CAC in 1974, various versions of a truly continuous annular chromatographic apparatus have been constructed and tested experimentally a t Oak Ridge National Laboratory (Scott et al., 1976; Canon and Sisson, 1978; Begovich et al., 1983). A number of applications have been considered systematically, including the separation of Ni, Cu, and Co in ammoniacal solutions (Begovich, 1982), the separation of iron and aluminum in ammonium sulfate-sulfuric acid solutions (Canon et al., 1980), the separation of hafnium from zirconium in sulfuric solutions, and the separation of waste cheese whey (Byers, 1982). The majority of the experiments conducted at ORNL have been carried out with Dowex 50W-X8, a polystyrenedivinylbenzene, strong-acid cation-exchange resin. In addition, size exclusion chromatography on model systems was investigated using Sephadex G15. A promising field of application of continuous annular chromatography is that of bioseparations. Recent advances in biology have created an enormous potential for revolutionizing developments in medicine, nutritional products, agriculture, specialty chemicals, and eventually the utilization of renewable energy sources and the production of commodity chemicals. However, in order to implement these developments in an industrial context, it is also necessary that advancements be made in the area of downstream processing. Development of effective and efficient separation techniques suitable for production scale applications is especially needed, as the economic impact
of these operations is frequently dominant in biotechnology applications. While it is clear that chromatography has a prominent role in this field, it is also apparent that many systems could be more efficiently handled by continuous processing. The glucose-fructose fractionation is a significant industrial problem, which, because of its scale of operation, benefits from continuous operation. Typical fructose syrup contains 42% fructose, 52% glucose, and 6% oligosaccharides on a dry weight basis. Because of the lower cost of fructose syrup for equivalent sweetness, it is used as a sucrose replacement in some foods and beverages. Since fructose is 1.3-1.8 times sweeter than sucrose and about 3 times sweeter than glucose and is more soluble in water at lower temperatures, syrup with a content of 55-97% fructose is desirable as a low-calorie sweetener for a number of applications. In addition, because of certain recognized short-term physiological effects, fructose is used as a sweetener for special dietary purposes. Because of a large demand for glucose-fructose fractionation, efficient adsorbent utilization and continuous operation are desirable. In the UOP-simulated moving bed Sorbex process (Broughton, 1966; Ching and Ruthven, 1984; Ching et al., 1985; Barker and Thawait, 1984,1986), an effective countercurrent operation is achieved by the sequential periodic movement of feed and exit ports through a series of fixed beds in the direction of fluid flow. Because of this periodic, countercurrent action, the adsorbent is used very efficiently in this process which has been successfully applied to the separaton of glucosefructose mixtures using the calcium form of ion-exchange resins and synthetic zeolites. On the other hand, the process is mechanically complex and may not be suitable for all scales of operations. Furthermore, it can completely separate only two components, with all other components accumulating as impurities in one of the two products. Multicomponent separations require a cascade of units. As an alternative, we have considered continuous annular chromatography, and we have investigated the separation of sucrose, glucose, and fructose mixtures using an ion-exchange resin in the calcium form. The investigation was conducted as a comparative study between conventional, batch chromatographic operation and continuous CAC operation. Separation of Sugars with Calcium-Form Resins Separation of sugars with calcium-exchanged, strongacid-type, cation-exchange resins occurs by virtue of the combined effects of size exclusion and restricted diffusion, as well as by virtue of specific interactions between calcium ions held by the resin and the hydroxyl groups of different sugars (Welstein and Sauer, 1984). The latter effects are sometimes referred to as “ligand exchange”, which occurs when water molecules held in the hydration sphere of calcium ions are exchanged for one or more hydroxyl groups of a polyol. Glucose and fructose, for example, can freely penetrate the matrix of resins with a 4-12% degree of cross-linking and a coordination complex with calcium ions may be formed depending upon the availability of properly spaced and oriented hydroxyl groups (Reeves, 1949). Thus, the conformation of glucose and fructose determines their relative affinity for the resin and their distribution coefficient. Both glucose and fructose are cyclic sugars (pyranoses) and exist as different anomers, the a and p forms, which are present in solution as an equilibrium mixture. For glucose, the mixture is about one-third a- and two-thirds fl-D-glucosetwhile 0-D-fructoseis the largely predominant form of fructose (Lehninger, 1977). Goulding (1975) has
Ind. Eng. Chem. Res., Vol. 27, No. 10, 1988 1875 pointed out that the ax-eq sequence of hydroxyl groups can form a relatively stable bidentate chelate with calcium ions and that @-D-glucosehas no ax-eq sequence, a-Dglucose and a-D-frUCtOSe have one such sequence, and @-D-fructosehas two. Thus, @-D-frUCtopyn”Seis the most strongly adsorbed species, and its distribution coefficient is typically about twice as large as that of a-D-glucose. Usually, a-D-fructose is present in negligible amounts since its equilibrium fraction is very small and the rate of mutarotation is high. On the other hand, @-D-glucoseinteracts only weakly with the resin, and since its rate of mutarotation is generally small compared to the speed of separation in a chromatographic column, it can be separated from its a-anomeric form. Sucrose and other higher molecular weight oligosaccharides interact only very weakly with the hydration sphere of calcium ions, and they are partially or totally excluded from the resin matrix owing to their larger molecular size.
Equipment Characterization of equilibrium, mass-transfer, and dispersive phenomena in chromatographic systems may be obtained from analyses of the dynamic behavior of a fixed bed in response to changes in solute concentration a t the bed inlet. For this purpose, a volume of the resin chosen for this study, Dowex 50W-X8, was sieved to recover a fraction in the 50-60-pm particle size range. This fraction was packed in a preparative scale, glass chromatographic column (1.5-cm diameter, 60 cm long) and converted to the calcium form with an aqueous solution of CaC12. Feed was provided to the top of the column by a set of two solvent delivery pumps for HPLC operation (Waters Model 501) controlled by a chromatographic controller. A 2-mL sample loop with an automatic injedor was available for the injection of chromatographic pulses. Detection of sugar concentrations in the bed effluent was obtained by means of a differential refractometer (Waters Model 401). Digital data were collected and stored by a microcomputer-based data acquisition system. Pulse injections of sugar solutions and tracers as well as breakthrough and elution experiments were carried out to characterize the system. Pulse sugar separation and tracer experiments were performed by injecting volumes of 25 pL, 1 mL, and 2 mL of solutions containing glucose, fructose, and sucrose in concentrations up to 25 g/L. The eluent flow rate was varied from 1to 3 mL/min. Positive or negative steps in solute concentrations, from 0 to 100 g/L, were used in breakthrough and elution experiments. Blue dextran, which because of its size (molecular weight = 2 000 000) does not penetrate the resin, was used as a tracer to provide estimates of dead volumes in the apparatus, the bed void fraction, e, and the hydrodynamic dispersion coefficient, D,. Average values for the distribution coefficients and mass transfer coefficients of the sugars could be calculated from the other experiments. Figure 2 is a schematic drawing of the construction details of the 279-mm-diameter CAC used in this study. The annulus is formed from two concentric open cylinders, both of which are flanged a t the bottom and constructed from clear Plexiglas. The inner cylinder is closed a t the top and serves as a spacer. The outer cylinder extends approximately 80 mm above the inner cylinder and is appropriately flanged and sealed. The resin is packed within the 1.27-cm-wide annulus to a level just below the top of the inner cylinder. A 30-40-mm-thick layer of 0.18-mm glass beads is placed on top of the resin to facilitate feed and eluent distribution. Eluent and feed are introduced through a stationary inlet distributor that ex-
/GAS
OVERPRESSURE
:SSURE SEA
FEED INLET
STREAMS
CONTINUOUS FEED STREAM ROTATING ANNULAR CHROMATOGRAPH
CONSTITUENTS
STATIONARY ELUENT WASTE COLLECTION
COLLECTION
ELUATE EXIT ANNULAR BED
Figure 2. Construction details of the CAC apparatus.
tends through the top flange and is sealed by two O-rings contained in the flange. The eluent liquid is delivered to the CAC by a pump which, in turn, is actuated by a conductivity-based level controller to maintain the liquid level in the head space above the layer of glass beads. Flow through the annular bed is regulated by the application of a pressure-controlled air blanket in the head space above the liquid level. A digitally controlled drive system is used to slowly rotate the entire assembly while the feed injection port remains stationary. Porous, high-density polyethylene plugs located at the top of each exit hole provide support for the sorbent bed while permitting the effluent to exit through 180 individual exit tubes. The separation performance of the CAC unit can be readily determined by connecting an in-line, continuous detection instrument with a single eluate exit tube. As the CAC unit rotates, the eluate exit tubes also rotate, and the detector receives eluate from all circumferential points as the apparatus completes a 360” turn. Thus, if the concentration-detection instrument is attached to a recording device, an apparently conventional (Le., concentration versus time) chromatogram is recorded. Angular concentration profiles are immediately obtained, considering that the angular displacement from the feed point, 8, is related to the recorded time, t , and to the rotation rate, w , by 8 = ut. A differential refractometer (Waters Model 401) was used for this purpose. The connection between one of the eluate exit tubes and this detector was provided by a digitally controlled peristaltic pump to compensate for the pressure drop through the instrument and ensure constant flow through the sample tube, equal to 11180 of the total eluent flow. The millivolt signal from the detector was converted to a digital signal by an AJD conversion board (Data Translation Model 2805) interfaced to a microcomputer (IBM-PC AT) for graphical display and digital data storage. The CAC was operated isocratically and the aqueous feed mixtures consisted of single-component and multicomponent solutions of glucose, fructose, and sucrose with concentrations up to 200 g/L in each sugar. Blue dextran was added to the feed mixtures as a tracer with a concentration of 0.5 g/L. The sugars used in this study were reagent-grade sucrose (Mallinckrodt), p-D-fructose (Sigma Chemical), a-D-glucose (Sigma Chemical), and Blue Dex-
1876 Ind. Eng. Chem. Res., Vol. 27, No. 10, 1988
tran 2000 (Pharmacia). The feed mixtures were made from these materials without further purification. The feed stream was pumped by a positive displacement pump (Altex Model llOA) to the top of the bed through a fixed feed nozzle, whose tip was located within the layer of glass beads. The level of the eluent liquid was maintained through use of the liquid-level controller that operated the eluent feed pump. A concentration of 0.5 g/L of CaC12was added to the eluent and feed in order to provide sufficient electrical conductivity for the level controller. The total flow rate through the bed was kept constant by the regulated air pressure at the top of the bed. Additional details of the equipment and experimental method are described by Howard (1987). ~~~
0
Results and Discussion Fixed Bed Operation. For a system for which the adsorption equilibrium may be described by a linear isotherm, the following equations may be used to describe transport of the solute within a plug-flow chromatogrpahic column:
(1 -
6)
at = koo(c -
i)
Equation 1 is a material balance for a solute within a differential element of the column, and eq 2 describes mass transfer to and from the solid phase in terms of a global interphase mass-transfer coefficient, koa. If a two-film mass-transfer resistance model is used, koa is related to the individual mass-transfer coefficients for external film transport, kp,and for transport within the resin particles, k ', by the following relationship (Ruthven, 1984): -1 =-
koa
1 kp
+ -k'K1
X
Here Q is the quantity of solute injected with the feed mixture per unit cross-sectional area of the fixed bed and Z = t - €Z/U. Pulse and step chromatographic experiments were conducted at room temperature in the fiied bed apparatus. As a typical example, the result of a 2-mL pulse injection of a 25 g/L of glucose and fructose mixture is shown in a t which the Figure 3. For each peak, the time, t,, maximum concentration occurs and the width of the chromatographic peak a t half peak height, A, were measured. From these values, the number of transfer units
1500
2000
TIME (a)
-GLUCOSE THEORETICAL PREDICTION
---- FRUCTOSE THEORETICAL PREDICTION -.-COMBINED
SUGARS EXPERIMENT
Figure 3. Comparison of experimental and calculated fixed bed chromatograms (concentration = 25 g/L of glucose and 25 g/L of fructose; QF = 3.25 mL/min). Glucose calculated values are the sum of the contributions from both optical isomers. Table I. Equilibrium Distribution a n d Mass-Transfer Coefficients (Dowex 50W-X8,Calcium Form, T = 25 "C) sugar K kna, 5-l kna/K, s-* sucrose 0.130 0.0042 0.032 fi-D-glUCOSe 0.228 0.0143 0.063 a-D-glucose 0.294 0.0184 0.063 p-D-frUCtOSe 0.657 0.0409 0.062
available in the column, n, and the distribution coefficients, K , for each sugar were calculated by using the following equations (Sherwood et al., 1975):
n = -koaz =
(3)
Note that adoption of a much more sophisticated masstransfer model is, in general, not warranted for linear systems. In this case, in fact, chromatographic response becomes very insensitive to the details of the mass-transfer model chosen, and a simple two-film model usually suffices. For a feed pulse of infinitesimal width, neglecting axial dispersion and invoking the usual experimentally realized assumption that the number of transfer units is larger than 5, the following approximate solution can be used to describe chromatographic response (Sherwood et al., 1975):
1000
500
U
16(ln 2)(
%)'
and
where
t,,
= ,,t
-
€2
U
In the case of glucose, the trace represents the partially resolved sum of the a- and fi-D-glucose sugars, which manifest themselves as a peak followed by a shoulder. Since both separation and equilibration of the two species occur simultaneously in the chromatograph, an exact mathematical model of the glucose separation is considerably more complex than is warranted in most studies. In this study, we assumed that equilibration was much slower than separation. On the basis of this assumption, K and n values were determined for the individual anomers from chromatographic peaks obtained experimentally. Because of the predominance of the fi form of fructose, only one peak was found for this material. An average value of t = 0.39 was found from the elution time of blue dextran. The average values of the mass-transfer coefficient, k&, and the distribution coefficient,K , are presented in Table I. Values for the a-anomeric form of fructose were not determined owing to its small concentration in an equi-
Ind. Eng. Chem. Res., Vol. 27, No. 10, 1988 1877 librium mixture. Since the K values found from the step concentration experiments were in very good agreement with those obtained from the pulse experiments, it is apparent that an essentially constant distribution coefficient may be assumed over the concentration range from 0 to a t least 100 g/L. This is in agreement with prior observations reported in the literature (Ching and Ruthven, 1984; Ching et al., 1985; Barker and Thawait, 1984) and with the anticipated behavior of the uptake of sugars by ligand exchange. Figure 3 shows a comparison between an experimental chromatogram and concentration profiles calculated according to eq 4 using the parameters values listed in Table I. The agreement shown appears to be within the experimental error of the method if one accounts for the fact that average values of K and koa were used in these calculations and not those optimum for this particular run. Further, the theoretical curve shown for glucose is the sum of the contributions from the two forms, and as we have previously discussed, this treatment is not rigorous. The relative importance of internal and external mass-transfer resistance merits some discussion. As seen in Table I, considerably different koa values were found for the different sugars. These values appeared to be approximately independet of flow rate. Since glucose and fructose are very similar in structure and have the same molecular weight, large differences in fluid-phase mass transport rates are not expected. Internal diffusional resistance is, thus, likely to dominate the transport process. For glucose and fructose, the global mass-transfer coefficient, koa, is observed to be approximately linearly dependent on the distribution coefficient, K . As shown by Table I, the ratio koa/K is approximately equal to 0.062 s? for these two sugars and is independent of concentration and fluid flow rate. The ratio k , p / K for sucrose is instead about one-half of the value found for glucose and fructose, suggesting that the controlling mass-transfer mechanism is that of restricted diffusion in the resin matrix. The value of k g for sucrose is also independent of concentration and fluid flow, a t low concentrations. CAC Operation. If concentration and fluid velocity gradients in the radial direction are ignored, a steady-state solute material balance for a rotating CAC in a fixed cylindrical coordinate system yields the following equation:
ED,- + az2 a2C
€De a 2 C ac - - = we - + w(1R: ae2 ae
E)
a9
ac
ae
aZ
- + tu -
(7)
with the solid transport rate equation given by
where D, and DBare the axial and angular dispersion coefficients, w is the rate of rotation, Ro is the radius of the annular chromatograph, and z and 8 are the axial and angular coordinates, respectively. A comparison of eq 7 and 8 for CAC operation with eq 1 and 2 for fixed bed operation shows a term-by-term coorespondence with the exception of the angular dispersion term in eq 7. In fact, if the angular dispersion term is considered negligible and if the transformation 8 = wt is made, the two sets of equations are identical. Thus, as also pointed out by Wankat (1977), under conditions of negligible angular dispersion, the unsteady-state, onedimensional chromatographic process is equivalent to the steady-state, two-dimensional CAC process. Equation 4 can be used as an approximate solution for the prediction of chromatographic response in CAC operation if both axial and angular dispersion are neglected.
Table 11. Base Conditions for CAC Operation "meter value bed length, cm 58 eluent" flow rate, L/h 4.0 rotation rate, deg/h 240 feedb rate, mL/min 1.0
,
i240
"Eluent, distilled water containing 0.5 g/L of CaC12. bFeed, 25 g/L of glucose, 25 g/L of fructose, and 0.5 g/L of blue dextran. DISPLACEMENT FROM FEED (DEG)
0)
i I 0
1O ;
1;O
2;O
FEED:25g/L OF G.F E L U E N T : 8.89 L l h R O T A T I O N 240'lh
4,O
I 600
1
I
I 1200
1800 2400 TIME ( 8 )
I
J
3000
3600
-THEORY - TOTAL (GLUCOSE + FRUCTOSE) ---- EXPERIMENT- COMBINED SUGARS Figure 4. Comparison between experimental and calculated concentration profiles for CAC operation at base conditions (Table 11); QT = 8.89 L/h.
In order to use this equation for CAC operation, one must replace .f with
and Q with CFUQF
360'
Q = -QT w
where C F is the solute concentration in the feed mixture, QF is the feed flow rate, and QT is the total flow rate of fluid through the annular bed. Continuous annular chromatography experiments were conducted by changing one parameter a t a time from the base operating conditions listed in Table 11. A typical CAC chromatogram is shown in Figure 4. Both the angular displacement coordinate and the corresponding time of elution in an equivalent fixed bed operation are shown in this graph. Zero time (or zero degrees) corresponds to the position directly below the stationary feed inlet. As shown by Figure 4, the blue dextran, glucose, and fructose were eluted a t increasing angular displacement from the feed point. The peak elution angle, 8, and the exit bandwidth, W ,were determined for each peak and then used to express the separation performance in terms of resolution, R , according to wz
e = -[t U + (1 - € ) K ]
(11)
and
R=
2(8, - 8,)
w 2 + Wl For the purpose of comparing experiments with theoretical
1878 Ind. Eng. Chem. Res., Vol. 27, No. 10, 1988 300
I
--
I
I
I
T
‘
I
A
0 GLUCOSE
240 m
-4
0 FRUCTOSE
c
THEORY
/
.
- . - . -. -.-.
0
-0
I i
I I
0 -
--
I
EXPERIMENT APPROX SOLUTION E X A C T SOLUTION
1
0 0
30
60
90
120
150
IS0
210
240
270
300
0
30
SO
90
ROTATION RATE (deg/h)
120
150
IS0
210
240
270
300
ROTATION RATE (deglh)
Figure 5. Effect of rotation rate on glucose and fructose peak positions (base conditions, Table 11). Comparison of experimental and calculated results.
Figure 7. Effect of rotation rate on glucose-fructose resolution (base conditions, Table 11). Comparison of experimental and calculated results.
I00
0 -0
8 5
1
1
w
r
1
GLUCOSE
0 FRUCTOSE
80 + -
-.-
P
APPROX SOLUTION EXACT SOLUTION
60-
E!
-0EXPERIMENT
-
-
THEORY
2
-
g
2
0
0-
2.0
2.5
40c
a
t W x
INITIAL BANDWIDTH’ 0
30
60
90
120
150
180
210
240
270
300
ROTATION RATE ( d e g l h )
Figure 6. Effect of rotation rate on exit bandwidths (base conditions, Table 11). Comparison of experimental and calculated results.
predictions in terms of glucose-fructose resolution, glucose was represented as a single component whose K value is the average of those for the CY and the p forms. Figure 4 also shows a comparison between experimental and calculated CAC concentration profiles. The parameter values obtained from fixed bed experiments listed in Table I were used for these calculations without further adjustments. The agreement between experimental results for CAC operation and numerical values calculated on the basis of data obtained from fixed bed experiments was quite good, although it was observed that fructose generally eluted before its predicted peak in CAC runs. This is likely due to the presence of calcium ions in solution, which were added only to the CAC eluent mixture. Overall, however, experimental and predicted runs were very similar, revealing that dispersive effects other than intraparticle mass-transfer resistance do not affect this separation significantly. The effect of rotation rate was studied in a series of experiments at the base conditions given in Table 11. This effect is important since decreasing the rotation rate increases the column loading and thus the capacity of the apparatus. The rotation rate was varied from 30 to 240°/h. The effect of rotation rate on peak position is shown in Figure 5. As is expected from eq 11,the peak position varies linearly with rotation rate for both sugars. Exit bandwidths also increase as rotation rate increases, as illustrated in Figure 6. At low rotation rates, the W values approach the width of the feed sector. Thus, the resolution
0 0
0.5
1 .o
1.5
3.0
F E E D F L O W RATE (mLlmin)
Figure 8. Effect of feed flow rate on glucose-fructose resolution (base conditions, Table 11). Comparison of experimental and calculated results.
decreases with decreasing rotation rate. Figure 7 shows this trend. At low rotation rates, the situation is analogous to that of an overloaded fixed bed chromatograph. The resolution values determined experimentally follow the theoretically predicted trend but have a slight negative deviation that may result from the adverse effects on equilibrium distribution of the presence of calcium ions in solution, as discussed earlier. The effects of feed rate on separation performance were studied at the base conditions over a flow rate range of 0.5-2.5 mL/min. The results are shown in Figure 8. The experimental R values are slightly lower than the theoretical predictions based on fixed bed data largely because of the slight shift of the 8 value for fructose. In this range of low flow rates, the feed may be regarded as infinitesimal and the resulting bandwidth is independent of loading, as may be seen from eq 4. A t considerably higher feed loadings, of course, the bandwidth would increase as the feed flow rate is increased and the separation performance would be reduced. Feed flow rates above 2.5 mL/min caused overflowing of the feed mixtures above the layer of glass beads with the existing feed distributor and, thus, could not be tested. A series of runs was instead performed in which eluent flow rate was varied from 2.1 t o 9.0 L / h while using the base conditions given in Table 11. Figure 9 shows the dependence of resolution on eluent velocity. The experimental resolution values are again slightly lower
Ind. Eng. Chem. Res., Vol. 27, No. 10, 1988 1879 3
I
1
I
I -0- EXPERIMENT
-
-
1
THEORY
i
30
-z2 0 4
20
c
z 0
g
10
90 0 0
I
1
I
I
I
5
10
15
20
x103
VELOCITY ( c m l s )
240
DISPLACEMENT FROM FEED (DEG) 0
120
60
240
180
300
360
S UC R 0SE 2.5
THEORY
ROTATION: 240'1h
2
-
2 -
I-
4.
+ Z
-
GLUCOSE
0
a
3
210
Figure 11. Comparison of experimental and predicted concentration profiles for CAC operation (concentration = 200 g/L of glucose and 200 g/L of fructose; QT = 7.53 L/h).
-0- EXPERIMENT
z 0 I-
180
DISPLACEMENT FROM FEED (DEG)
Figure 9. Effect of superficial eluent velocity on glucose-fructose resolution (base conditions, Table 11). Comparison of experimental and calculated results.
-
150
120
1.5
W
0
z
0
0.5
0 0
L 0
I 900
I
1
1800
2700
TIME
I 3600
I
I
4500
5400
(8)
0 0
25 FEED
50
75
100
CONCENTRATION ( g l L )
THEORY
---- EXPERIMENT-COMBINED SUGARS
Figure 10. Effect of feed solute concentration on glucose-fructose resolution (base conditions, Table 11). Comparison of experimental and calculated results.
Figure 12. Comparison of experimental and predicted concentration profiles for CAC operation at base conditions (Table 11); QT = 4.72 L/h.
than the theoretical curve because of the combined effects of a somewhat lower fructose retention and increased hydrodynamic dispersion. The effects of feed concentration of glucose and fructose were also studied over the concentration range of 5-100 g/L in each component, with 0.5 g/L of blue dextran. The experimental resolution values were nearly constant as the feed sugar concentration was varied, as shown in Figure 10. If one of the CAC runs is selected as the basis for the computation of K, the internal consistency of the theoretical treatment is closely followed. A slight decline is observed a t higher sugar concentrations, perhaps as a consequence of slight nonlinearity of the equilibrium isotherm (Barker and Thawait, 1984). A number of experiments was also carried out with feed mixtures containing 200 g/L in each sugar. In this case, the product was too concentrated to be analyzed directly with the differential refractometer. Thus, for these runs,samples were collected a t the outlet of the CAC unit and analyzed by HPLC. Figure 11 shows experimental and calculated concentration profiles for the separation of a mixture containing 200 g/L of glucose and fructose. At this concentration, there is a slight decline in adsorption capacity and, more importantly, a considerable increase in viscosity that leads to somewhat greater peak dispersion, especially for fructose (Barker and Thawait, 1984). The calculated profiles were
based on the parameter values given in Table 11. The calculated and experimental values of the glucose-fructose resolution in this run were 1.3 and 0.8, respectively. Multicomponent separations were also investigated in CAC operation, as illustrated, for example, in Figure 12 for a feed solution of 25 g/L each of glucose, fructose, and sucrose and 0.5 g/L of blue dextran. Concentration profides for steady-state CAC operation, predicted on the basis of fixed bed data, are also shown in this figure. Elution positions and shapes of the peaks for the run agree quite well with those of the theoretical curve. Resolution values for this particular run are about 1.5 for glucose-fructose and about 0.6 for sucrose-glucose. The dextran peaks eluted both from the fixed bed and CAC apparatus provided a measure of the hdyrodynamic dispersion coefficient, D,. The results of these analyses are illustrated in Figure 13, in the form of Peclet number as a function of Reynolds number. The solid line is based on the following correlation given by Butt (1980): #e = 0.2 + 0.0iiRe0~48 (13) Note that, in the low particle Reynolds numbers that are characteristic of the experiments in this series and are typical of chromatographic operations, the Peclet number is essentially constant and the dispersion coefficient, D,, is a linear function of the product of u, the superficial velocity in the bed, and d,, the particle diameter. In the
1880 Ind. Eng. Chem. Res., Vol. 27, No. 10, 1988 '
O
w
-
FEED
-
200
0 C A C RUNS
0 F I X E D BED
. _I
100
ape
/
= 0.2
+ 0.01 1
01
(Re)' 4 8
Y
z
II +
100
z W z
0
:1-I
10-210-3
0 10''
10'2
0
60
120
180
240
300
360
DISPLACEMENT (degrees)
Re
Figure 13. Experimental and calculated dependence of Peclet number on Reynolds number for fixed bed and CAC experiments.
case of the CAC experiments, no attempt was made to distinguish between angular and axial dispersion; thus, for these experiments, D,is used as a global parameter, representative of both dispersive effects. This approach is obviously not rigorous, but it provides a reasonable basis for comparison with axial dispersion information available for fixed beds. As shown in Figure 13, the fixed bed data agree very well with the correlation, but the CAC data deviate from the solid line. Although the scatter of data is considerable, the results appear to indicate that somewhat greater dispersion exists in CAC operation, a result that might be attributed not only to the DBcontribution, but also to the hydrodynamics and geometric complexity of the feed and exit ports. Calculations of chromatographic peaks for the sugar separation system which included the axial dispersion term were found nevertheless to be essentially undistinguishable from the calculations based on the nondispersed plug-flow model. Qualitatively, it was observed that under equivalent operating conditions the fructose-glucose resolution was, on the average, about 20% lower for the CAC than values calculated from the fixed bed parameters. Furthermore, the fructose peak shape was more dispersed than the fmed bed based computation. In addition to the reasons cited above, one might consider differences in column packing methods and wall effects as contributing to the differences in behavior of the two chromatographic systems. CAC Scale-Up. Scaling up the CAC separation system involves the conception of an optimum design. As demonstrated by previous experiments, the linear chromatographic model may be used for sugar concentrations of 200 g/L with reasonable accuracy. To permit the simulation of operating conditions typical of production scale, a solution of the model equations which takes into account the finite size of feed pulses, in the case of a fixed bed system, or the finite size of the feed sector, in the case of CAC operation, is needed. The following series solution has been obtained by Carta (1988) and may be conveniently used for these calculations:
+
U ( t2Fj m tt E )
Figure 14. Calculated concentration profile for CAC operation at high feed throughput (concentration = 200 g/L of glucose and 200 g/L of fructose; QF = 2.8 L/h; QT = 9.2 L/h; w = 600 deg/h; z = 60 cm).
where t F is the time during which feed is applied to the column, t E is the period of elution, and r is given by
The series solution is uniformly and rapidly convergent. For calculations for typical chromatographic conditions, 5-10 terms are generally sufficient. For CAC operation, the feed duration, t F , and the elution period, t E , are replaced by the sector of circumference over which the feed is applied and by the sector of circumference between consecutive feed sectors, respectively. The time, t , is replaced by e/u. As an example, calculations for a feed concentration of 200 g/L, a feed flow rate of 2.8 L/h, an eluent flow rate of 9.2 L/h, and a rotation rate of 600deg/h have been carried out and are shown in Figure 14. The feed solution would occupy about 83 deg, and product would be withdraw from a sector of about 300 deg a t the bottom of the annular bed. For these conditions, if two product fractions are collected as indicated by the vertical dashed line in Figure 14,90% of the fructose fed could be recovered with a purity of about 90%. It should be pointed out that these calculations neglect the effects associated with the increased viscosity of very concentrated sugar solutions. Although these effects may be mitigated by operating a t higher temperatures, a reduction in the achievable separation performance may nevertheless be expected. The final point to be considered in scaling up is the effect of an increased annulus width. Radial dispersion effects appear to be negligible in the sugar separation system, primarily because of the dominance of intraparticle diffusional resistance which largely determines the spreading of chromatographic peaks. Flow distribution could be a significant problem, as one needs to distribute the feed stream uniformly over a sector of the bed annulus. However, Begovich et al. (1983) have demonstrated experimentally that up to 96.7% of the volume of a 27.9cm-diameter CAC unit could be packed with no appreciable loss of resolution, for the separation of copper and nickel using Dowex 50W-X8. A properly designed distribution system was constructed to ensure even flow of feed and eluent throughout the packing. On the grounds of these observations, scaling up the separation shown in Figure 14 to a 10-ft-diameter CAC unit, 95% packed with adsorbent, should provide a throughput of about 1.8 m3/h with the same separation performance. It should be noted,
E]} (14)
- j 2 + r2
Ind. Eng. Chem. Res., Vol. 27, No. 10, 1988 1881 however, that in practice larger particle sizes may be used, resulting in an increased column length.
Warren G. Sisson and the helpful suggestions of Steven Bridges are greatly appreciated.
Conclusions The separation of glucose, fructose, and sucrose mixtures using calcium-loaded Dowex 50W-X8 cation-exchange resin and distilled water eluent has been performed in both a fixed bed column and a CAC unit. Equilibrium and mass-transfer parameters were determined in the fiied bed column. The effects of feed rate, feed concentration, eluent velocity, and rotation rate on the performance of the rotating chromatograph were studied. Dispersive masstransfer effects were studied in both systems, and quantitative data from both apparatus were modeled and compared. The following conclusions can be drawn from the experimental and theoretical work conducted: 1. Operation of the CAC for the separation and purification of binary and multicomponent aqueous sugar mixtures has been demonstrated. Separations are obtained with continuous throughput with the apparatus operated a t steady state. The mechanical simplicity of the concept lends itself to effective scale-up. Experiments on the separation of mixtures of glucose and fructose reported here indicate that essentially total separation may be obtained continuously, even at high concentrations. 2. The CAC unit developed yields results that are in approximate agreement with theoretical predictions based on fixed bed operation. Thus, the separation performance can be easily predicted on the basis of conventional fixed bed chromatographic experiments. 3. Tracer experiments with blue dextran indicated, as expected, that increased dispersion exists in CAC. However, for the system investigated, under typical chromatographic conditions, the effects of axial and angular dispersion are negligible as compared to the dispersion of chromatographic peaks resulting from intraparticle mass-transfer resistance. 4. For the system investigated, the apparent lack of interactions between glucose and fructose in solution and the lack of competition for adsorption sites on the ionexchange resin (at least up to 200 g/L of total sugar in the feed) allow the use of the simple linear chromatographic theory for the prediction of CAC performance. Intraparticle mass transfer is simply represented with a solid film mass-transfer parameter which is related to the intraparticle diffusivity by the Glueckauf approximation (Glueckauf, 1955). Only data obtained from fixed bed experiments are required t o implement the model. As has been shown in earlier studies at ORNL, the CAC adds the very significant advantage of continuous operation to chromatographic processes. The separation performance of CAC is inherently inferior to that of other continuous or semicontinuous countercurrent sorption processes, such as the UOP Sorbex process, when masstransfer resistances are limiting. On the other hand, however, contrary to the Sorbex process, the CAC retains the ability to carry out multicomponent separations which is the hallmark of chromatography. The continuous chromatographic separation of sugars on the calcium form of Dowex 50W-X8 resin is an interesting application of this concept.
Nomenclature
Acknowledgment This research was sponsored, in part, by the Office of Industrial Programs, U S . Department of Energy, under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, inc. The experimental participation of
a =
interfacial area, cm-l
c = liquid-phase solute concentration, g/L cF = feed solution concentration, g/L
d, = particle diameter, cm D , = dispersion coefficient, cm2/s Do = dispersion coefficient, cm2/s
k ’ = solid-phase mass-transfer parameter, s-l kf = fluid-phase mass-tranfer coefficient, cm/s ko = global mass-transfer coefficient, cm/s K = equilibrium distribution coefficient n = number of transfer units [ k o a z / u ] Pe = Peclet number [d,u/D,] q = solid-phase average solute concentration, g/L Q = column loading of solute, g/cm2 Q F = feed flow rate, mL/min Q T = total flow rate, L/h R = resolution, defined in eq 1 2 Ro = radius of annular bed, cm Re = Reynolds number [d,u/v] t = time, s tE = elution period, s tF = feed period, s t^ = chromatographic time, s fmm = peak chromatographic elution time, s u = superficial velocity, cm/s u = interstitial velocity, cm/s W = exit bandwidth, deg z = bed axial position, cm Greek Symbols A = time interval at half of peak maximum concentration, s e = bed void fraction 0 = displacement from feed point, deg 8 = peak elution angle, deg v = kinematic viscosity, cm2/s ?r
= numerical value
w
= rotation rate, deg/h
Registry No. Dowex 50W-X8, 11119-67-8; sucrose, 57-50-1; glucose, 50-99-7; fructose, 57-48-7; dextran, 9004-54-0. Literature Cited Barker, P. E.; Thawait, S. J . Chromatogr. 1984, 295, 479. Barker, P. E.; Thawait, S. Chem. Eng. Res. Deu. 1986, 64, 302. Begovich, J. M. Ph.D. Dissertation, The University of Tennessee, Knoxville, 1982. Begovich, J. M.; Byers, C. H.; Sisson, W. G. Sep. Sci. Technol. 1983, 18, 1167. Broughton, D. B. US Patent 3 291 726.13, 1966. Butt, J. B. Reaction Kinetics and Reactor Design; Prentice-Hall: Englewood Cliffs NJ, 1980; p 274. Byers, C. H. 1980-81 Report, MIT School of Chemical Engineering Practice, Cambridge, MA, 1982. Canon, R. M.; Sisson, W. G. J. Liq. Chromatogr. 1978,1, 427. Canon, R. M.; Begovich, J. M.; Sisson, W. G. Sep. Sci. Technol. 1980, 15, 655. Carta, G . Chem. Eng. Sci. 1988, in press. Ching, C. B.; Ruthven, D. M. Can. J. Chem. Eng. 1984, 62, 398. Ching, C. B.; Ruthven, D. M.; Hidajat, K. Chem. Eng. Sci. 1985,40, 1411. Dinelli, D.; Polezzo, S.; Taramasso, M. J. Chromatogr. 1962, 7,477. Giddings, J. C. Anal. Chem. 1962, 34, 37. Glueckauf, E. Trans. Faraday SOC.1955,51, 1540. Goulding, R. W. J . Chromatogr. 1975, 103, 229. Howard, A. J. M.S. Thesis, University of Virginia, Charlottesville, 1987. Lehninger, A, L. Biochemistry;Worth: New York, 1977; pp 277-298. Martin, A. J. P. Discuss. Faraday SOC.1949, 7, 332. Reeves, R. E. J . Am. Chem. SOC.1949, 71, 215. Ruthven, D. M. Principles of Adsorption and Adsorption Processes; Wiley: New York, 1984; pp 235-250.
1882
Ind. Eng. Chem. Res. 1988,27, 1882-1886
Scott, C. D.; Spence, R. D.; Sisson, W. G. J . Chromutogr. 1976,126, 381. Sherwood, R. K.; Pigford, R. L.; Wilke, C. R. Muss Transfer; McGraw-Hill: New York, 1975; pp 548-591. Wankat, P. C. AIChE J . 1977, 23, 859.
Welstein, H.; Sauer, C. In Ion Exchange Technology; Naden, D., Streat, M., Eds.; Ellis Harwood: London, 1984; pp 463-471.
Receiued for reuiew March 14, 1988 Accepted June 21, 1988
Separation of Alcohols from Water by Adsorption on Cross-linked Polymethacrylic Ester Containing a Pyridinium Group Nariyoshi Kawabata,* Yoshiyuki Sumiyama, and Naoki Matsuura Department of Chemistry, Faculty of Engineering and Design, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan
Cross-linked polymethacrylic ester containing a pyridinium group showed selective adsorption of alcohols in aqueous solution. Incorporation of a pyridinium group, N-benzyl-4-vinylpyridinium bromide, into cross-linked polymethacrylic ester enhanced the capacity of the polymer for ethanol adsorption, probably because of the hydrophilicity of the pyridinium group. However, cross-linked poly(N-benzyl-4-vinylpyridinium bromide) showed poor ability to selectively adsorb alcohols. The adsorption capacity was determined by the flow column method a t 30 "C. The total capacity for adsorption of several alcohols by the polymer, when compared under optimal conditions, is given in parentheses in units of milligram/gram: methanol (93), ethanol (112), propyl alcohol (148), isopropyl alcohol (132), butyl alcohol (283), and tert-butyl alcohol (156). Hydrophobic interaction between the alkyl group of each alcohol and the surface of the polymer, as well as a steric effect, is suggested as an important factor in selective alcohol adsorption. Adsorbed alcohols were easily elute-d by a purge of nitrogen a t 78 "C. The production of alcohol by fermentation of renewable plant biomass has often been anticipated as a future alternative to dependence on petroleum for liquid fhels. However, a major economic problem associated with preparation of alcohols from a fermentation broth is the high energy expenditure required for the separation of alcohol from the fermentation broth. The usual distillation process consumes more than 50% of the energy required for ethanol production (Ghose and Tyagi, 1979; Malik et al., 1983; Aldridge et al., 1984). Various alternative methods of ethanol separation have been investigated in recent years. A possible means of accomplishing such separation is selective adsorption on solid adsorbents. Anion- and cation-exchange resins (Malik et al., 1983; Sinegra and Carta, 1987), synthetic hydrophobic resins (Lee et al., 1982; Pitt et al., 1983; Lencki et al., 1983; Malik et al., 19831, biomass materials (Ladisch and Dyck, 1979; Hong et al., 1982; Ladisch et al., 1984; Rebar et al., 1984), activated carbon (Bui et al., 1985),silicate (Milestone and Bibby, 1981; Bui et al., 1985), zeolite (Bui et al., 19851, and molecular sieves (Pitt et al., 1983; Teo and Ruthven, 1986) are used as the adsorbents. An attractive feature of synthetic resins is that they can be manufactured with surfaces of predetermined characteristics, making it possible to design a resin for a specific adsorption application. Malik et al. (1983) and Sinegra and Carta (1987) described promising results using commercial weak-acid and strong-acid cation-exchange resins, but the ion-exchange function is not necessary for separation of alcohol from water. However, a problem with the use of synthetic hydrophobic resins in separation of alcohol from water is their poor affinity for water, which limits satisfactory contact of the resin surface with the aqueous solution, thereby limiting the capacity of the resin to achieve complete separation of alcohol from water. In the present study, we have investigated the effect of incorporation of a hydrophilic functional group into synthetic hydrophobic
resins on their ability to separate alcohol from water. Cross-linked polymethacrylic ester was selected as the base for the synthetic hydrophobic resin because the polymer is less hydrophobic than styrene-divinylbenzeneresins and because Lencki et al. (1983) have reported promising results with ethanol separation using a cross-linked acrylic ester resin. A pyridinium group was selected as the hydrophilic functional group, because cross-linked poly(vinylpyridinium halide) showed sufficient hydrophilicity and did not display any ion exchange with inorganic salts (Kawabata and Morigaki, 1980). Experimental Section Cross-linked polymethacrylic esters containing a pyridinium group were prepared by copolymerizationof methyl methacrylate, ethylene glycol dimethacrylate, and 4vinylpyridine, followed by conversion of the pyridyl group into a pyridinium group by reaction with benzyl bromide. Polymerizations were carried out in a 5-L round-bottomed, three-necked flask equipped with a mechanical stirrer, a reflux condenser, and a nitrogen inlet. Polymerization was carried out in a 1.5 w t % aqueous solution of poly(viny1 alcohol) with a degree of polymerization of 2000 at 80 " C for 5-6 h using 0.5 mol % of 2,2'-azobis(isobutyronitri1e) as the initiator, under a nitrogen atmosphere. Reaction of the copolymer beads with benzyl bromide was performed in ethanol at 80 "C for 5-6 h. The characteristics of the cross-linked polymethacrylic ester containing the pyridinium group are listed in Table I. The surface area of MAP-8 determined by standard BET method was 44.7 m2/g. Cross-linked poly(N-benzyl-4-vinylpyridinium bromide) was prepared in a similar manner by copolymerization of 4-vinylpyridine with divinylbenzene, followed by reaction with equimolar amounts of benzyl bromide. Amberlite IRC-50 provided by Rohm and Haas Co., Philadelphia, PA, was used in the H form for comparison. The amount of the pyridinium group contained
0888-5885/88/2627-1882$01.50/00 1988 American Chemical Society
